1. Field of the invention
The present invention relates to a propeller nozzle, particularly for a missile or rocket, in which troublesome lateral forces can occur.
This type of nozzle ejecting a propulsive gaseous fluid at the rear of a missle, consists of a forward convergent portion, a minimum intermediate section, called a throat, and a rearward divergent portion coupled to the minimum throat section and having a rearwardly increasing section, with respect to the direction of missile movement.
2. Description of the Prior Art
The operation of highly superexpanded nozzles results in more or less considerable gaseous fluid separation in the divergent portion and, with certain relatively low expansion rate values, the separation is of a dissymetric and sometimes unstable nature.
During the separation phases of missile stages, this nozzle operation is characteristic of the priming phase during which the propeller pressure is progressively established in a few milliseconds. The presence of the base of the lower stage tank close to the outlet of the nozzle is an additional cause for dissymetric fluid separation owing to equipments which are distributed asymmetrically on it; the same applies to possible turning of the nozzle and when the two stages are no longer in line. When the nozzle has a high cross-sectional ratio, the dissymetric fluid separation phenomenon results in a sometimes considerable lateral force which slightly and temporarily modifies the stability of the upper part of the missile and which can produce forces prohibitive to the structure of the nozzle and the control actuators of the nozzle.
The same problem is encountered in launcher nozzles whose divergent portion outlet i.e., the cross-sectional exit area of the nozzle, is optimized to obtain the best possible performance over the whole of the launcher trajectory and which can operate in a superexpanded state at low altitudes. For example, in U.S. Pat. No. 3,237,402 is described a variable thrust nozzle for a missile in which the exit area of the nozzle gradually increases as the pressure of the atmosphere outside of the nozzle decreases so as to substantially achieve optimum thrust at all altitudes. The gradual increase of the nozzle exit area is carried out by the use of several removable smooth ring-shaped ramps that are disposed concentrically within the divergent portion and arranged sequentially from front to rear thereof with the rearward end of the forward ramp contiguous the forward end of the ramp immediately to the rear thereof. Each of the ramps is ejected at a predetermined time dependant on the exhaust gas pressure and, therefore, on the gas temperature communicated to the ramp, by means of ignition of explosive attaching bolts thereby increasing the exit area of the nozzle to the size of the ramp in line. The ramps have a "continuous" task during the ascension of the missile that is, typically, a few minutes, 2 to 5 minutes approximately, depending on the required flight altitude corresponding to the stabilized flight of the missile.
Thus, in such a nozzle, the forward ramp in combination with the convergent portion and the throat portion is equivalent to a nozzle having a short smooth divergent portion at the start of the propeller pressure increase, i.e., for 10 to 50 ms approximately, and the problems of lateral forces in the nozzle during this start phase are not solved.
Observations and experimental measurements have shown that a nozzle is subjected to lateral forces, all the greater when the ratio of the divergent portion outlet section to the throat section is high. These forces briefly develop at the start of the propeller pressure increase. Although these forces have very limited consequences on the stability of the missile, owing to the fact that they are brief, they are very important where the design and construction of the nozzles and piloting devices associated with the missile are concerned. In the case of a missile with separable stages, these lateral forces develop when the ratio of the internal pressure of the gases in the propeller becomes some 2 to 10 times that in the intermediate stage of the missile.
These lateral forces are due to a separation or break-down of the gaseous fluid exhausting from the propeller and which is located in the nozzle, when it is superexpanded. The position of the separation depends on the ratio of the pressure in the propeller to the pressure downstream from the nozzle, i.e., in the rearward end of the nozzle. Wind tunnel tests have confirmed that, when this pressure ratio is between about 2 and 10, the fluid separation is dissymetric and/or unstable. The fluid separation occurs just downstream from the throat, at the start of the nozzle divergent portion, and creates lateral forces which are higher when the surface affected by the separation is considerable.
It should be observed that the affected surface is great and consequently, the lateral forces are considerable for long nozzle divergent portions with a high section ratio. These nozzles are frequently being considered for future missiles which have to supply increasingly high initial acceleration to reduce the altitudes in which the stage separations take place. Consequently, the pressure prevailing in the propellers of such missiles must be increasingly high, and the nozzles have increasingly large sizes in order to maintain correct thrust and pressure adaptations, thus the ratio of the sections of the nozzle divergent portions becomes higher. It is therefore necessary to remedy the considerable lateral forces to which these nozzles are subjected.